The present invention relates generally to ablator compositions and more particularly to low density yet highly durable ablator compositions and methods for mixing the compositions and forming ablative thermal protection systems.
Ablative materials have been used in a variety of applications to protect and insulate structure that is subjected to extreme thermal conditions. For example, many aerospace vehicles that traverse, exit, and enter the atmosphere of the Earth travel at high velocities, and as a result, their exterior aerosurfaces, and to some degree their substructure, experience high aerothermal loads. Aerothermal loads have been managed using a variety of techniques including insulation, radiant cooling, active cooling, conduction and convective cooling, and by phase change or ablative materials. Generally, ablative materials are applied to the affected aerosurfaces to absorb the extreme heat in order to insulate the vehicle from the thermal environment.
The thermal management technique of ablation has been widely used for a variety of applications since the early 1930s. Ablative materials were used in early rocket systems for nose cap protection and have also been used as re-entry heat shields on the Gemini and Apollo space vehicles, and further on many modern rocket nozzles. Many of these materials, although suitable for use in the aforementioned applications, have handling and longevity issues that preclude application on a system that is subjected to frequent handling and that may be stored for several years prior to use.
Known ablative materials comprise a variety of constituent components, each at certain percentages by weight or volume, to achieve the desired level of thermal protection and other physical properties. Generally, ablator compositions are a composite material comprising a resin matrix with a variety of filler materials to reduce the overall density or provide other physical properties. For example, U.S. Pat. No. 4,031,059 to Strauss discloses low density ablator compositions that employ an elastomeric silicone resin with a variety of filler materials including ground cork, silica or glass microspheres, phenolic microballoons, and silica and/or carbon fibers. The compositions of Strauss, however, specifically limit the density to a range of 0.20 g/cc (12.10 lb/ft3) to 0.30 g/cc (19.00 lb/ft3), and more specifically, the density of an RF (radio frequency) transparent composition to 0.25 g/cc (15.55 lb/ft3).
Unfortunately, the lower density ablator compositions of the known art, especially those near 0.24 g/cc (15.55 lb/ft3) such as Strauss, have low abrasion resistance and are easily damaged and worn during handling. As a result, the fragile nature of lower density ablator compositions requires special handling after fabrication, which, in many instances, includes custom packaging for shipment and an attendant increase in cost.
The ratio of filler to resin, by weight or volume, is relatively high in the known art, e.g. 2.2:1 by weight in Strauss. With higher ratios of filler to resin, plausible methods of forming an ablative structure are somewhat limited, such as the closed-die molding process disclosed in Strauss. As an example, a large volume of fibers can clog or severely limit the flow of ablator composition flowing from a nozzle using spraying methods. Although an increased amount of fillers can reduce the overall density or improve mechanical properties of the ablator composition for increased performance, the trade-off with available forming methods presents limitations in terms of producing lower cost structures.
Additional known art ablator compositions have included a variety of other constituent elements such as metal fillers, colloidal clay fillers, boron and oxygen compounds, polyurethane resins, a mixture of both epoxy and polysulfide resins, and many others too numerous to detail herein. The known art compositions, however, include numerous fillers to achieve a desired set of properties such as thermal, mechanical, and others. As a result, such compositions may be costly and difficult to fabricate with a relatively large number and variety of fillers. In addition, many known art ablator compositions demonstrate relatively low thermal and abrasion resistance performance under high heat flux and pressure loads observed in Mach 6 to 8 vehicles.
Accordingly, there remains a need in the art for an ablator composition that is of low density yet has high abrasion resistance and durability before, during, and after high thermal loads, and which is relatively low cost and simple to fabricate.
In one preferred form, the present invention provides an abrasion resistant and more durable, low density ablator composition that generally comprises silica microballoons embedded in a silicone resin. The ablator composition further comprises a catalyst to cause crosslinking of the polymer chains in the base silicone resin, and a thinning fluid to control the viscosity of the composition.
The resulting ablator composition has a low density, approximately 0.32 g/cc (20.74 lb/ft3), compared with known art low density ablator compositions. Further, the ablator composition of the present invention can withstand temperatures up to approximately 1,760xc2x0 C. (3200xc2x0 F.) while ablating slowly. In addition, the composition has low thermal conductivity, is RF transparent so as to not impede signal transmissions to and from the vehicle, and has high abrasion resistance and durability.
In other preferred forms, the present invention provides methods for mixing the ablator composition and forming ablative structures. The methods generally include the steps of mixing the individual constituent components of the ablator composition in prescribed percentages by weight and in a particular order, as more fully described hereinafter, and forming the composition into a final part or structure. The forming methods include, but are not limited to, manual application to the structure, open or closed die molding, spraying, and extrusion. Depending on the application method used, the percentages of constituent components of the ablator composition will vary accordingly.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.